Both the body louse (Pediculus humanus humanus) and the head louse (P. humanus capitis) are obligatory human ectoparasites. The body louse is a serious public health threat because it transmits a variety of human diseases whereas the head lice causes one of the most prevalent human infestations, pediculosis. Recent genome analysis revealed that both body and head lice have one of the smallest insect genomes (~108 Mb). Comparison of transcriptional profiles uncovered that almost the same number of genes was annotated both in the head louse (10,770 genes) and the body louse (10,771 genes) among 10,775 protein-coding genes predicted from the body louse genome. Despite their similar genetic background, body and head lice have several differences in their biological features, such as choice of habitat on human host, body size and vector competence. Annotation of major detoxification genes revealed that they are dramatically reduced in human lice compared to other insects except for the honey bee and that, despite the overall reduction in number, human lice retain at least a minimum repertoire of genes known to confer metabolic or toxicokinetic resistance to insecticides, suggesting their high potential for resistance development. Comparison of insecticide target site gene sequences and transcription levels of detoxification genes enabled the identification of toxicodynamic and metabolic factors of insecticide resistance and further allowed the development of molecular markers for resistance detection. Transcriptional profiling during tolerance was used to identify ivermectinmetabolizing detoxification genes, indicating that such an approach may allow proactive resistance management. Comparison of genomes and transcriptomes between body and head lice suggested that vector competence difference is not attributed to the difference in the composition of immune related genes but rather to their transcriptional regulation and/or not-yet-identified epigenetic factors.

The body and head lice (Pediculus humanus humanus and Pediculus humanus capitis, respectively) are hematophagous ectoparasites of humans and only the body louse between two is known to transmit three bacterial diseases through its feces. The proliferation profiles of Bartonella quintana, the causative agent of trench fever, inside the louse body and its excretion patterns were investigated in the two louse subspecies following oral challenge with B. quintana-infected blood meal. The initial density of B. quintana was sustained inside head lice without any noticeable proliferation for the entire period after infection. In contrast, B. quintana proliferated rapidly inside body lice and the maximum density reached at 10 days post-infection. The numbers of bacteria detected in feces from infected lice were almost the same and steadily decreased over time in both body and head lice. Nevertheless, the viability of the bacteria, as determined by fluorescence, was significantly higher in body louse feces, especially at 1 day post-infection and this tendency lasted for 11 days. These findings suggest that excretion of feces containing more viable B. quintana that is proliferated inside body lice following ingestion of infected blood meal is responsible for the higher vector competence of body lice.

The human body and head louse are ectoparasites of humans for thousands of years. Although both body and head lice belong to a single species, Pediculus humanus, only body lice are known to transmit several bacterial diseases. This different vector competence is assumed to be due to their different immune responses. Here, the immune reactions in the alimentary canal were investigated in both two louse subspecies following oral challenge of Escherichia coli as a model gram-negative bacteria. In propagation assay, head lice suppressed the proliferation of E. coli in their epithelial cells effectively at the early stage of infection, resulting in gradual reduction of E. coli number in gut tissues. In contrast, the number of E. coli steadily increased in gut tissues of body lice. No apparent alteration of transcription was observed following E. coli challenge in three important genes for the humoral immune responses, PGRP as a recognition gene and defensin1 and 2 as effector genes. Nevertheless, the basal transcription levels of these genes were higher in the gut tissues of head lice than body lice. Considering that there is no cellular immune reactions in gut tissues, these findings suggest that the higher constitutive transcription levels of major immune genes in head lice can contribute to their initial defense and immune capacity against intestinal bacterial infection.

Body and head lice (Pediculus humanus humanus and Pediculus humanus capitis, respectively) are typical ectoparasites of humans. They differ not only in the ecological habitat but also in the vector competence in spite of their conspecific nature. Only body lice transmit several bacterial pathogens to humans, including Bartonella quintana, Rickettsia prowazekii and Borrelia recurrentis. In this study, the proliferation rates of two model bacteria, a gram positive Staphylococcus aureus and a gram negative Escherichia coli, were determined following bacterial challenge by cuticular injection. Both bacteria proliferated rapidly in body lice at the early stage of bacterial challenge but not in head lice, suggesting that head lice have more sensitive immune responses to these bacteria. In vivo phagocytosis assay revealed that head lice have much higher phagocytic activity against E. coli than body lice whereas only slight differences in phagocytic activity against S. aureus were observed between the two lice species. Taken together, these findings suggest that the reduced phagocytosis activity of body lice contributes, at least in part, to their higher vector competence.

To search for hyper-variable genetic markers that can distinguish regional populations of head lice, we screened the inter simple sequence repeats (ISSR) based on the genome database of body louse, which is closely related conspecific species. An ISSR mining software, SciRoKo 3.4, was employed to excavate ISSR markers from the genome database under the MISA mode (≥ 60 bp repeats). Entire body louse genome (ca.100 Mb) was loaded to SciRoKo for ISSRs mining. A total of 5,336 ISSRs were obtained, and primers specific to individual ISSRs were designed by the Primer 3 and DesignPrimer 1.0 softwares. In order to prove the compatibility of body louse ISSRs to head lice, 31 PCR primers were randomly chosen out of a total of 613 pairs, and their appropriateness was tested by comparing the amplified PCR band patterns between body and head lice. Eleven primer pairs that resulted in poor or little amplification were excluded, and 20 primer pairs were further tested for three head louse populations (California, Panama and Chung-ju, Korea). Finally, nine primer pairs ensuring robust amplification of highly variable band patterns were selected to use for population genetic study of head lice.

Body lice (Pediculus humanus humanus), obligatory human ectopasites, differ from conspecific head lice (Pediculus humanus capitas) in the choice of habitat and the capacity of disease transmission. Only body lice are known to naturally transmit a variety of human diseases, including epidemic typhus, trench fever and relapsing fever. Such differences in vectoral capacity are expected to be due to their differences in immune responses during pathogen invasion. Here, we annotated 94 immune related genes from the body louse genome and determined the differences in the transcription profiling of immune related genes between the head and body lice by qrt-PCR. In general, head louse females showed more sensitive immune responses than body louse females to Staphylococcu. aureus dermal challenge as judged by selective induction of defensin 2 in head lice. In contrast, when the 3rd nymphs were orally challenged, body lice exhibited more sensitive immune responses than head louse to Escherichia coli as judged by selective induction of defensin 1 and PGRP in body lice. These stage- and pathogen-specific differences in immune responses should provide basic insight on the vector competencies in the head and body lice.

A quantitative sequencing (QS) protocol that detects the frequencies of sodium channel mutations (M815I, T917I and L920F) responsible for knockdown resistance in permethrin-resistant head lice was tested as a population genotyping method. Genomic DNA fragments of the sodium channel α-subunit gene that encompass the three mutation sites were PCR-amplified from individual head lice with either resistant or susceptible genotypes, and combined together in various ratios to generate standard DNA template mixtures for QS. Following sequencing, the signal ratios between resistant and susceptible nucleotides were calculated and plotted against the corresponding resistance allele frequencies. Quadratic regression coefficients of the plots were close to 1, demonstrating that QS is highly reliable for the prediction of resistance allele frequencies. Prediction of resistance allele frequencies by QS in several globally collected lice samples including 12 Korean lice populations suggested that permethrin resistance varied substantially amongst different geographical regions. Three local populations of Korean lice were determined to have 9.8-36.7% resistance allele frequencies, indicating that an urgent resistance management is needed. QS should serve as a preliminary resistance monitoring tool for proper management strategies by allowing early resistance detection.